Ya, you can never completely rid your tank of ich. It is impossible, it is ALWAYS there. You just don't see it if everything is hunky-dory. When a fish gets stressed, it's immune system becomes lowered. The ick parasite recognizes this and attacks the host, hoping to spread. But ick spores are all throughout the rocks and everywhere in your tank.
The best way to control it is good water params and resolving stress issues in the display.